1. Field of the Invention
This invention relates to transistor devices and more specifically to MOS transistor devices capable of operating in the VHF and UHF regions.
2. Description of The Prior Art
MOS transistors are well known in the prior art. One such MOS transistor 11 useful in the VHF region is shown in FIG. 1. Formed on N type silicon substrate 10 is N type epitaxial silicon layer 20. Formed within epitaxial silicon layer 20 is P type region 22 which serves as the body of the transistor of FIG. 1. Located within body region 22 is N type region 23 which serves as the source. Located above and insulated from source region 23 and body region 22 by gate insulation 25 is gate electrode 24. Making electrical contact with source 23 and body region 22 is source electrode 26, allowing electrical connection between body 22 and source 23 to external devices (not shown). The substrate 10 serves as the drain of the transistor.
In response to a gate voltage applied to gate electrode 24 which is greater than the threshold voltage V.sub.t of transistor 11 (i.e., the voltage required to be applied to gate electrode 24 in order to cause transistor 11 to turn on), a sufficient number of electrons are attracted to channel region 29 (having length L), thereby allowing current to flow between source region 23 and drain 10 through channel 29 and epitaxial silicon region 20. Conversely, with a voltage less than the threshold voltage V.sub.t applied to the gate electrode 24, sufficient electrons will not be attracted to channel 29, thereby preventing current flow between source region 23 and drain 10 through channel 29 and epitaxial silicon region 20.
MOS transistor 11 of FIG. 1 is widely used for high frequency power switching devices. Gate electrode 24 typically comprises polycrystalline silicon which, because of its relatively high sheet resistivity of approximately 30 ohms per square, limits the operating frequency of transistor 11 to a maximum frequency of approximately 1 to 2 MHz, which is well below the very high frequency (VHF) and ultrahigh frequency (UHF) frequency ranges, which typically extend as high as 300 MHz, and in excess of 1,000 MHz, respectively. When gate electrode 24 comprises a metal, for example aluminum or an alloy thereof, the sheet resistivity of gate electrode 24 is quite low. In this event, the operating frequency range of transistor 11 is limited by the input and output capacitances of transistor 11. The output capacitance of transistor 11 is caused by the capacitance formed across the PN junction between P type body region 22 and N type epitaxial region 20. The power gain bandwidth (i.e., the band of frequencies over which the power gain meets or exceeds a desired value) is inversely proportional to the output capacitance. Accordingly, it is desirable to minimize the output capacitance of a transistor in order to increase its power gain bandwidth. The output capacitance of MOS transistor 11 is approximately 7 pF for each centimeter of channel width (with the width of channel 29 being measured perpendicular to the cross-sectional view of FIG. 1).
The input capacitance of MOS transistor 11 of FIG. 1 is caused by the overlap of gate electrode 24 and source region 23. Thus, the greater the amount of overlap between gate electrode 24 and source 23, the greater the input capacitance of MOS transistor 11. The maximum frequency of operation f.sub.t is directly proportional to the transconductance g.sub.m and inversely proportional to the input capacitance C.sub.in : EQU f.sub.t .varies.g.sub.m /C.sub.in (1)
where
f.sub.t =the maximum operating frequency; PA1 g.sub.m =the transconductance; and PA1 C.sub.in =the input capacitance. PA1 W=the channel width (measured perpendicular to the cross-sectional views shown in the Figures); PA1 .mu..sub.n =the electron mobility in the channel; and PA1 L=the channel length.
Furthermore, EQU g.sub.m .varies.W.mu..sub.n /L (2)
where
Thus, since C.sub.in .varies.L, it follows from equations (1) and (2), EQU f.sub.t .varies..mu..sub.n /L.sup.2. (3)
Thus, the greater the input capacitance of MOS transistor 11, the lower the operating frequency of transistor 11. A typical input capacitance for a MOS transistor 11 is approximately 12 pF for each centimeter of channel width.
Another prior art MOS transistor 121 is shown in the cross sectional view of FIG. 2. Formed in P type substrate 10 is N type source region 32 and N type drain region 31. Surrounding drain region 31 is N type drift region 30 having a relatively low dopant concentration. Located above and insulated from source region 32, drain region 31, drift region 30 and substrate 10 by gate insulation 25 is gate electrode 24. When a voltage greater than the threshold voltage V.sub.t of transistor 121 is applied to gate electrode 24 via gate terminal 27, electrons are attracted to the channel region 41 (having length L), thereby allowing current to flow between source region 32 and drain region 31 through the channel 41 and drift region 30. Conversely, with a voltage less than the threshold voltage V.sub.t applied to gate electrode 24 via gate terminal 27, sufficient electrons will not be attracted to the channel 41, thereby preventing current flow between source region 32 and drain region 31.
Another prior art MOS transistor 131 suitable for use at VHF is depicted in the cross sectional view of FIG. 3. N type substrate 10 serves as the drain of the device. Formed on N type substrate 10 is N type epitaxial silicon layer 20 of relatively low dopant concentration. Formed within epitaxial layer 20 is P type body region 40 and formed within P type body region 40 is N type source 41. A V type groove 17 is cut into N type source 41, P type body region 40, and partially into N type epitaxial region 20, as shown. A layer of insulation 21 is then formed on the surface of the device, including the surface of the V type groove 17. Source electrode 26 and gate electrode 42 are then formed allowing electrical connection from gate 42 and source 41, to external devices (not shown). When a voltage greater than the threshold voltage V.sub.t of transistor 131 is applied to gate electrode 42 via gate terminal 27, a sufficient number of electrons are attracted to the channel region 49 (having length L) to cause channel 49 to become sufficiently N type to allow current to flow between source region 41 and drain region 10 through channel 49 and N type epitaxial region 20. Conversely, when a voltage less than the threshold voltage V.sub.t of transistor 131 is applied to gate electrode 42, the channel region 49 remains sufficiently P type to prevent current flow between N type source 41 and N type drain 10.
Yet another prior art MOS transistor 140 suitable for use in the VHF region is shown in the cross-sectional view of FIG. 4. This structure is also described in an article by Oakes, et al. entitled "A Power Silicon Microwave MOS Transistor", IEEE Transactions on Microwave Theory and Techniques, Volume MTT-24, No. 6, June, 1976, pages 305-311, which is hereby incorporated by reference. The transistor of FIG. 4 includes N type substrate 40 which serves as the drain of the device, and gold drain contact 48 applied to the backside of substrate 40, thereby allowing low resistivity electrical connection to the drain 40. The transistor of FIG. 4 also includes N type drift region 41 of relatively low dopant concentration, P type body region 60 containing channel region 42, P+ region 44 which provides electrical contact to body region 60, and N+ source region 43. A groove is cut thereby exposing the surface of N type source region 43, P type body region 60, and N type drift region 41. A thin layer of gate oxide 49 is then formed on these exposed surfaces, and a gate electrode 45 is formed adjacent to, but insulated from, channel region 42.
While the Oakes, et al., structure of FIG. 4 is suitable for use in the VHF region, the Oakes structure is extremely difficult to fabricate because the wafer must be precisely oriented with respect to the direction from which the metal is deposited in order to allow the metalization to form gate region 45 at the proper location adjacent to channel region 42. Thus, the angle .zeta. between the source oxide 47 and the gate metalization 45 must be precisely controlled. Furthermore, the distance D between adjacent source oxide regions 47 must be precisely controlled in order to allow proper formation of the metal gate region 45.
Another disadvantage of the Oakes, et al., structure of FIG. 4 is that the channel region 42 between N type source region 43 and drift region 41 is located primarily on the &lt;111&gt; silicon surface, due to the isotropic etching used to expose the surfaces of source region 43, channel region 42, and drift region 41 prior to formation of gate oxide 49. The &lt;111&gt; surface provides a lower electron mobility (i.e., typically 450 cm.sup.2 /volt-second) and thus a lower transconductance in the channel region than if the channel region 42 were formed on the &lt;100&gt; crystal surface, which has a typical electron mobility of 650 cm.sup.2 /volt-second. See, for example "Electron mobility in Inversion and Accumulation Layers on Thermally Oxidized Silicon Surfaces", S. C. Sun and J. D. Plummer, IEEE Transactions on Electron Devices, ED-27, 8, 1980 (pp. 1497-1508), which is hereby incorporated by reference. As previously described, the maximum operating frequency f.sub.t is directly proportional to the transconductance g.sub.m, and thus the lower transconductance provided by the &lt;111&gt; surface as compared with the &lt;100&gt; surface limits the maximum operating frequency.
Another disadvantage in the Oakes, et al., structure of FIG. 4 is that the body region 42 must be sufficiently wide to allow both the N type source 43 and the P type body contact diffusion 44 to be separated from each other and yet contacted by the source metalization 46. This requirement means that the Oakes, et al. transistor must be of a rather large size, and thus have a rather high parasitic output capacitance because, as previously described, the output capacitance is caused by the capacitance provided by the PN junction formed between P type body region and N type drift region 41. This high output capacitance limits the power gain bandwidth of transistor 140, as previously described.
Another prior art MOS transistor 150 suitable for use in the VHF region is shown in the cross-sectional view of FIG. 5. The structure of FIG. 5 includes N type silicon substrate 49 which serves as the drain, and N type epitaxial layer 50, P type body region 60 which contains channel region 73, P type body contact region 61, and N type source region 63. A layer of oxide 64 is formed on the surface of the device and a V type groove 37 is formed, thereby to expose the surface of N type source region 63 and P type body region 60. A gate oxide layer 62 is formed on the surface of the groove. Metal is subsequently applied thereby forming source electrode 66 and gate electrode 65. Channel region 73 is located in the portion of body region 60 adjacent to gate oxide layer 62. When a gate voltage is applied to gate electrode 65 which is greater than the threshold V.sub.t of transistor 150, a sufficient number of electrons are attracted to the channel region 73 to allow current to flow between source 63 and drain 49 through channel 73, and epitaxial layer 50. Conversely, when a gate voltage is applied to gate electrode 65 which is less than the threshold V.sub.t of the transistor 150, an insufficient number of electrons are attracted to channel region 73, thereby preventing current flow between source 63 and drain 49.
The transistor 150 of FIG. 5 has several disadvantages. Due to the use of an anisotropic etch to form V groove 37, the channel region 73 is formed on the &lt;111&gt; silicon surface, thereby resulting in lower electron mobility and thus lower transconductance than if a channel were formed on the &lt;100&gt; silicon surface, thereby providing a low maximum operating frequency f.sub.t, as previously described in conjunction with transistor 140 of FIG. 4. Furthermore, the body region 60 must be sufficiently wide between adjacent V grooves to allow both source region 63 and body contact region 61 to be contacted by the source metalization 66. This results in a rather large cell size (typically 23 microns between the lowest points of adjacent interdigitated grooves) and thus a rather large parasitic output capacitance caused by the PN junction formed between P type body region 60 and N type drain 50. This output capacitance is typically 8 pF for each centimeter of channel width (channel width being measured in a direction perpendicular to the cross-sectional view of FIG. 5). As previously described, such a large output capacitance also provides a low power gain bandwidth for transistor 150.
Bipolar transistors are also used in the VHF and UHF regions. However, such bipolar transistors have disadvantages, including their susceptibility to thermal runaway.